This is an illegal act that Walmart believes they are above the law and still tries to enforce.

It is unlawful detention.

You can only stop someone after they've left the store, and if you saw them steal an item. Just like if I saw you steal my wallet in front of me, I can stop you and get it back. If I think you have something of mine, but have no proof, then I can't detain you.

If you detain someone who hasn't stolen anything, then you are violating their rights.

If a Walmart employee tried to detain me, and I hadn't stolen anything, then it is within my right to defend my right to travel. The person attempting the detention is initiating violence/force, and you would be responding with violence/force.